A radio communication system based on the CDMA scheme is a communication scheme characterized in that interference between communication terminals is reduced by assigning codes with orthogonality to the respective communication terminals and multiplying communication signals by the codes. On the other hand, recent mobile communication systems have been standardized to support not only speech communication but also data communication of packets and the like. In a mobile communication system based on the CDMA scheme, power per bit can be increased by changing the spreading ratio (=chip rate after spreading/bit rate before spreading) of an orthogonal code in accordance with the bit rate at which communication is performed, and decreasing the spreading ratio and performing code multiplexing with respect to communication terminals which are communicating with each other at a higher bit rate.
As shown in FIG. 8, an adaptive antenna base station apparatus in the above conventional mobile communication system comprises a base station control unit 1, baseband processing unit 8, radio transmitting/receiving units 31 to 3n, and transmission/reception antennas 41 to 4n. The base station control unit 1 is connected to the constituent elements of the base station apparatus (including the baseband processing unit 8, the radio transmitting/receiving units 31 to 3n, and constituent elements (not shown) which performs speech and data processing and the like) except for the transmission/reception antennas 41 to 4n, and has a function of interfacing with a communication/host station apparatus (not shown) for communication of control/monitoring data. The baseband processing unit 8 is connected to the transmission/reception antennas 41 to 4n through the radio transmitting/receiving units 31 to 3n, and has a function of performing encoding processing and decoding processing for communication data transmitted/received to/from a communication terminal (not shown) through the transmission/reception antennas 41 to 4n. That is, the baseband processing unit 8 comprises an encoding processing unit 81 which performs encoding computation for transmission data, a decoding processing unit 82 which performs decoding computation for reception data, and an amplitude/phase control unit 83 which performs amplitude/phase computation for communication data for directivity control on the adaptive antennas. Each of the radio transmitting/receiving units 31 to 3n is connected to a corresponding one of the transmission/reception antennas 41 to 4n and has a function of converting data from the baseband processing unit 8 into data in the radio frequency band and transmitting it through a corresponding one of the transmission/reception antennas 41 to 4n, and a function of converting a signal in the radio frequency band received from a corresponding one of the transmission/reception antennas 41 to 4n into a signal in the baseband and outputting it to the baseband processing unit 8.
With regard to the above adaptive antenna base station apparatus, studies have been made on an arrangement in which the downlink directing characteristics are controlled in accordance with the uplink communication error rate or reception power or the downlink error rate or the like reported from a communication terminal (e.g., “W-CDMA Mobile Communication Systems, Second Chapter: Radio Transmission Scheme”, edited by Keiji Tachikawa, Maruzen, Jun. 25, 2001, pp. 79-86). In addition, a method of improving interference characteristics from a high-power, high-speed transmission user to a low-speed transmission user by array antenna directivity control on a downlink channel based on the CDMA scheme has been proposed (see, e.g., Japanese Patent Laid-Open No. 2002-246970: pp. 5-9, FIG. 1). According to this method, after a despreading unit despreads an uplink channel spread-spectrum signal from a mobile station (communication terminal), a radio wave arrival direction estimating means estimates the direction of the mobile station, and an individual transmission power determining means determines transmission power from the data transmission speed determined by a data transmission speed determining means for the mobile station. A side-lobe level setting means determines a side-lobe level suppression value with respect to a main lobe from the data transmission speed ratio or spread spectrum ratio between high-speed transmission and low-speed transmission, and a null setting means suppresses interference near the main lobe. A transmission directivity pattern forming means then determines a transmission pattern.
In a conventional radio communication system, however, owing to the characteristics of radio communication, the propagation conditions between a base station and a communication terminal always change, and hence it is difficult to stably maintain perfect orthogonality between communication channels. In practice, this often causes interference in communication with other communication terminals. In a radio communication system based on the CDMA scheme, therefore, an increase/decrease in the communication capacity of the system is greatly influenced by how such interference is reduced. In addition, in the conventional radio communication system, the spreading ratio of orthogonal codes is changed in accordance with the bit rate at which communication is performed, and the spreading ratio of a communication terminal which is performing communication at a higher bit rate is decreased to perform code multiplexing. In the conventional radio communication system, therefore, the influence (interference) of one communication terminal with a high communication bit rate on the system is higher than that of a low-speed speech terminal. As a result, when communication terminals with high communication bit rates concentrate on one area (e.g., a sector), the capacity in the area may reach its upper limit although the number of communication terminals in the area looks small. This problem cannot be solved even by the technique in the above patent reference.